Electrical ladder-type filter



April 25, 1957 w. POSCHENRIEDER ET AL 3,316,510

ELECTRICAL LADDER-TYPE FILTER Original Filed April 1, 1963 2 she s- 1Fig.1

PRIOR ART Fig.2

April 1967 w. POSCHENRIEDER ET AL 3,316,510

ELECTRICAL LADDER-TYPE FILTER Original Filed April 1, 1963 2Sheets-Sheet 2 Fig.5

Fig 9 United States Patent Ofitice 3,315,5lfi Patented Apr. 25, 1967 7Claims. (31. 333-42 This application is a continuation of Ser. No.269,526, filed Apr. 1, 1963, now abandoned.

The invention disclosed herein is concerned with an electricalladder-type filter, comprising at least one transverse branch containingan electromechanical oscillator at which appears, as seen in theladderat the terminals of the transverse branch, in addition to the seriesresonance of the electromechanical oscillator, which produces anattenuation pole, at least one parallel resonance frequency (repetitionfrequency) lying in the filter barrier range, the action of which iscompensated by an attenuation produced in preceding and/or succeedingfilter elements.

A ladder circuit of particular construction is frequently being used atthe present time for making low pass filters and band pass filters withquartzes or other electromechanical oscillators. It is important inconnection with these ladder circuits, on the one hand, that anelectromechanical oscillator for the production of an attenuation poleis used in the transverse branch of the filter, and on the other hand,that so-called repetition poles appear directly adjacent to both sidesof the branch containing the oscillator, that is, barrier points withsubstantially the same resonance frequency which produce, however, onlyone operating attenuation pole which coincides at least approximatelywith the parallel resonance appearing in the transverse branch and lyingin the filter barrier range. The result obtained by this ladder circuitresides in that the structure of the electromechanical oscillator,especially the oscillator quartz, appears in the electrical equivalentor analog filter circuit in the desired manner.

It is in the design of low pass filters and broad band pass filters inthe form of such ladder circuits desirable to obtain a value as great aspossible for the ratio of static to dynamic capacity of the mechanicalfilter that is being used, for example, a quartz, and to provide therepetition pole in a suitable spacing from the attenuation pole producedby the oscillator, so that the losses and the barrier attenuation at thebarrier border in the pass range of the filter are substantiallydetermined by the oscillator and not by the elements which produce therepetition pole.

The problem and object of the invention reside in improving theproperties of such ladder circuits, primarily so as to produce aparticularly favorable ratio of static to dynamic capacity of theoscillator used, which may be a piezoelectric oscillator.

This object is realized, in connection with an electrical ladder-typefilter having at least one transverse branch containing anelectromechanical oscillator, in the branch circuit of which appears, asseen from the terminals of the transverse branch, the series resonanceof the electromechanical oscillator, which produces an attenuation pole,and in addition thereto at least one parallel resonance frequency(repetition frequency) lying in the filter barrier range, the action ofwhich is compensated by an attenuation produced in preceding and/orsucceeding filter elements, by effecting the compensation, in accordancewith the invention, in the form of a parallel resonance circuit in alongitudinal branch, or in the form of a series resonance circuit in atransverse branch, and providing this compensation in the branch circuitoutside of the filter section which is determined by the transverseelement containing the electromechanical oscillator and the longitudinalelements directly connected therewith.

'An advantageous embodiment for the realization of a low pass filter orof a band pass filter with steeply rising attenuation flanks, isobtained by using in the transverse branch, as an electromechanicaloscillator, a piezoelectric element, preferably an oscillating quartz.Further advantageous embodiments of the invention are obtained byeffecting the compensation by two parallel resonance circuits withidentical intrinsic resonance fre quency, whereby one such resonancecircuit is allocated to a preceding and the other to a succeedinglongitudinal branch, or by effecting the compensation by two seriesresonance circuits with identical intrinsic resonance frequency, whichresonance circuits are arranged in transverse branches, whereby one suchseries resonance circuit is disposed ahead of the transverse elementcontaining the oscillating quartz while the other in disposed followingthis transverse element.

It is moreover advantageous to effect the compensation with the aid of aseries resonance circuit in a transverse branch and a parallel resonancecircuit with the same intrinsic resonance frequency in a longitudinalbranch of the arrangement, whereby the series resonance circuit isdisposed ahead of the transverse element containing the oscillatingquartz while the parallel resonance circuit is disposed following thistransverse element, or to effect the compensation with the aid of aparallel resonance circuit disposed in a longitudinal branch and aseries resonance circuit with the same intrinsic resonance frequencydisposed in a transverse branch, whereby the parallel resonance circuitis disposed ahead of the transverse element containing the oscillatingquartz while the series resonance circuit is disposed following thistransverse element.

According to another feature of the invention, an advantageousembodiment is obtained by using in the transverse branch, with the aidof a dual circuit, a magnetostrictive element in place of thepiezoelectric element.

Further details of the invention will appear from the description ofembodiments which is rendered below with reference to the accompanyingdrawings.

FIG. 1 shows a portion of a known ladder-type band pass filter;

FIG. 2 indicates equivalent circuits for transforming elements shown inFIG. 1;

FIG. 3 represents the result of the transformation according to FIG. 2;

FIG. 4 indicates another ladder-type circuit obtained by transformationof elements shown in FIG. 3;

FIG. 5 shows equivalent circuits for effecting a trans forrnation withrespect to FIG. 4;

FIG. 6 illustrates a circuit obtained by transformation with the aid ofthe elements shown in FIG. 5; and

FIGS. 7 to 10 illustrate embodiments of the invention in which two-portnets are connected between the transverse branch containing themechanical oscillator and respective resonance circuits which may beeither parallel resonance circuits or series resonance circuits.

Referring now to FIG. 1 which shows, as noted above a portion of a knownladder-type band pass filter, wherein an attenuation pole is produced atthe point jm of the attenuation diagram, by the series resonance of thepiezoelectric oscillator indicated by a straight arrow. The

quartz is represented by its electrical equivalence or analog circuit,namely, by the element L C and G In the transverse branch of thiscircuit, in which the piezoelectric oscillator is disposed in parallelwith an inductance L there appears a parallel resonance frequency f,-

which is indicated by a semicircular arrow. This resonance frequency f'would produce an attenuation breakthrough since the correspondingtransverse branch becomes high ohmic at a frequency 72;, practicallycausing a through-connection between the .circuit 3 and the circuit 4. gIn order to avoid this undesired operation, there are provided, at bothsides of the transverse branch which is here being considered, twoparallel resonance circuits 1 and 2, in the longitudinal branch of thecircuit, the intrinsic frequency of which is identical and indicated byfa These two parallel circuits produce at the frequency fang, thesocalled repetition frequency, a common attenuation pole, the socalledrepetition pole. The parallel circuits 1 and 2 which produce therepetition pole, are di- 7 mensioned so that their intrinsic resonancefrequency f coincides approximately with the parallel resonancefrequency f of the transverse branch referred to (f f whereby theapparent input resistance of the circuit assumes at the point 3:12.again approximately the value infinity, thereby avoiding an attenuationbreak-through at this point. a

As may be seen from FIG. 1, the arrangement also contains in thelongitudinal branch, the parallel resonance circuits 3 and 4, suchcircuits producing further attenuation poles at the frequencies fw and fas Well as the parallel resonance circuits 5 and 6 in transversebranches, which aredimensioned according to further requirements posedfor the filter. The dash lines indicate that there may be providedfurther circuit elements at the respec.

tive ends of the ladder, which is, however, unimportant in connectionwith the following considerations.

The bracketed two-port net VPI and VPII which embrace, respectively, theelements ofthe resonance circuits 5, 1 and 2,6, are now reformed withthe aid of equivalent circuits, known per se, as shown in FIG. 2. (Thecircuit a of FIG. 2 merges with the aid of an ideal transformer with atransformation ratio lzu, into the circuit b, and vice versa.) The useof this transformation'in connection with the two-port nets VPI and VPII'(FIG. 1) results in the circuit shown in FIG; 3. As will be seen fromFIG. 3, the branch containing the quartz as well as the parallelresonance circuits 3 and 4, are not affected by the transformation. Anattenuation pole is. with a piezoelectrical oscillator also formed inthis circuit, at the point fs which is again indicated by a straightarrow. 'The parallel resonance circuits 1 and 2.

which produce at the frequency fm the repetition pole are not any more.directly connected with the transverse branch which is provided with theoscillator, but are separated by the parallel circuits 7 and 8, theintrinsic frequency of which lies according to the transformation withthe elements of FIG. 2, likewise at the frequency fm The repetitionfrequency fwQ of the parallel resonance circuits 1 and 2 coincide in thecircuit according to FIG. 3 at least substantially with the parallelresonance frequency f lying in the filter barrier range of thetransverse branch containing the oscillation quartz (fx mg). The circuittransformation entailed addition of the capacitor C and C and of theideal transformers A and B.

The two-port nets VPIII and WW, bracketed in' FIG. 3,-which embracerespectively the elements from the parallel resonance circuits 7, 3 and8, 4, are now again trans,-

formed with the aid of the equivalent circuits shown in FIG. 2, thusresulting in the ladder-type circuit repreresented'in- FIG. 4. The idealtransformers resulting in the transformation are thereby unimportant andfor the sake of clarity have been omitted in FIG. 4, that is,

the transformers are placed at'the input or the output I of the circuit,which as is known merely signifies respectively a multiplication of allinductivities or a division of all capacitances of the respectivecircuit section, with the value u. As will beseen, the parallelresonance circuits land 2, which produce the repetition pole, are notaffected by the transformation and the intrinsic resonance frequencylies now as before at the frequency fa The 7 addition of capacitors C7to C capacitors C and C likewise appear in the circuit. The capacitors Cand C are added by the transformation. The parallel circuits 3 and 4with the intrinsic frequencies fm and fa; also appear again in thelongitudinal branch of the circuit.

Upon measuring at the terminals K and K of FIG. 4 of the transversebranch containing the oscillation quartz, for example, with the aid ofan impedance meter, the apparent resistance depending upon thefrequencywhereby the input and the output of the filter can beterminated with real resistances, the value of which issuitably equal tothe wave impedance of the filterthere will appear zero points andinfinity points in the course of the apparent resistance. One of theinfinity points corresponds to the repetition frequency fang, to whichare tuned the parallel resonance circuits 1 and 2 which are disposed inthe longitudinal branch and produce the repetitionpole.

As may also be seen from FIG. 4, the circuit transformation results inthe addition of capacitors C and C as well as the coils L and L Sincethe capacitances C and C are due to the parallel connection to theoriginal quartz capacitance C additive, the ratio of the staticcapacitance which now consists of C +C +C to the dynamic capacitance Cis increased, which is extraordinarly advantageous for the realizationof the oscillation quartz. The newly added elements C C and L which aredisposed parallel with the original quartz, effect moreover a shiftingof the original parallel resonance frequency f of the transverse branchcontaining the oscillation quartzconsidering such transverse branch byitselfto a new value f which is in FIG. 4 again indicated by asemicircular arrow. The parallel resonance frequency 3, lies as a ruleadjacent to the repetition frequency or adjacent to one of the otherpole frequencies 12. or fa so that the finite attenuation based uponthese attenuation that the repetition pole can also be realized with theaid ofseries resonance circuits which are disposed in transversebranches of the arrangement. The bracketed twoport nets VPV and VPVIwhich consist of half-elements comprising respectively the, capacitor Cand parallel circuit 1 and the capacitor C and parallel circuit 2, arefor this purpose transformed with the aid of known equivalent circuitsaccording to FIG. 5. The corresponding transformation results in thecircuit shown in FIG. 6;

As will be seen from FIG. 6, the capacitors C and C; as well as theparallel resonance circuits 3 and 4, remain unchanged by thetransformation, that is, they remain as they also appear in FIG. 4. Thecapacitances C C and C of FIG. 4 are combined to form the static quartzcapacitance C and there applies the relation The inductivities L L and Lof FIG. 4 are combined to form the inductivity L according to therelation The parallel resonance frequency f appearing in the transversebranch containing the oscillation-quartz, thus remains preserved.However, the transformation effects formation of the repetition pole atthe point ja by the series resonance circuits 9' and 10 which lie intransverse branches of the arrangement and cause at the frequency npractically a shunt, such con-v dition being indicated by straightarrows. The circuit transformation results, as compared with FIG. 4, inthe Upon measuring the apparent resistance at the terminals K and K ofthe, transverse branch containing the oscillating quartz, there willagain appear an infinity point in the course of the apparent resistance,at the repetition frequency fs Various embodiments of the inventionutilizing the principle of repetition poles are again represented inFIGS. 7 to 10,

In each of these circuits there is provided in a transverse branch anelectromechanical oscillator Q, whereby an inductivity L and a balancingcapacitor C, which may be connected in parallel therewith. Two-port netsD and E are respectively disposed at the sides of the transverse branchwhich contains the quartz. The repetition pole is in FIG. 7 produced bythe parallel resonance circuits 1 and 2 (see also FIG. 4), which aredisposed in the longitudinal branch of the circuit. The paralleloscillation circuit 1 is connected ahead of the two-port net D and theparallel oscillation circuit 2 is disposed serially following thetwo-port net E.

In FIG. 8, the repetition ole is produced by two series resonancecircuits 9 and which are disposed in transverse branches (see also FIG.6). The series oscillation circuit 9 is disposed ahead of two-port net Dand the series oscillation circuit 10 is disposed serially following thetwo-port net E.

In FIG. 9, the repetition pole is produced by the series resonancecircuit 11 lying in a transverse branch and by a arallel resonancecircuit 12 lying in a longitudinal branch. The series circuit 11 isdisposed ahead of the two-port net D while the parallel circuit 12 isdisposed serially following the two-port net E. The two oscillationcircuits 11 and 12 are tuned to the identical resonance frequency fraz-In FIG. 10, the repetition pole is produced by a parallel oscillatorcircuit 13 disposed ahead of the two-port net D and by a seriesoscillation circuit 14 disposed serially following the two-port net E.

The following particularly characteristic features result in connectionwith the circuits according to FIGS. 4 to 6 as well as in the circuitsaccording to FIGS. 7 to 10:

(1) At least one attenuation pole is produced with an electromechanicaloscillator in ladder circuit.

(2) At both sides of the branch containing the electromechanicaloscillator, but not directly contiguous thereto, are disposedcombinations of reactance elements which produce an attenuation pole atthe identical resonance frequency (repetition pole).

(3) Upon measuring the apparent input resistance of the entire circuit,at the transverse branch which contains the electromechanicaloscillator, there appear a number of pole points and zero points. One ofthese pole points coincides with the frequency of the repetition poleswhich lies in the barrier range.

(4) Upon cutting the circuits according to FIGS. 4 and 6 and thecircuits according to FIGS. 7 to 10, at the points K /K and Kg/Kg, thusobtaining three partial two-port nets in which the reactance elementswhich pro duce the repetition pole do not any more belong to the partialtwo-port net which contains the electromechanical oscillator, andmeasuring the apparent input resistance at the terminal pairs of therespective partial two-port net which contains the electromechanicaloscillator, the apparent input resistance becomes zero at the resonancefrequency 72. of the repetition pole, when the repetition pole isproduced by series resonance circuits or, respectively, infinitely high,when the repetition pole is produced by parallel oscillation circuits.

These four characteristic features will be fully preserved when therepetition pole is produced by bridge circuits or the like, within theladder circuit.

Two band pass filter circuits (FIGS. 4 and 6) have been explained tobring out the invention more clearly, such circuits having, disposed ina transverse branch, an oscillating quartz, and being obtained bycircuit transformation with the aid of equivalence circuits. Thecircuits according to FIGS. 4 and 6 as well as those accord- 6 ing toFIGS. 7 to 10 can 'be respectively calculated or designed according tothe image parameter theory or according to the insertion loss theory, bya corresponding analysis of the circuit elements of a matrix accordingto the principle of the repetition poles.

There is, moreover, in connection with all circuits according to theinvention, the possibility of using in the transverse branch, with theaid of dual circuits, magnetostrictive elements in place of thepiezoelectric elements.

Changes may be made within the scope and spirit of the appended claimswhich define what is believed to be new and desired to have protected byLetters Patent.

The invention claimed is:

-1. An electrical ladder-type filter circuit having an electromechanicaloscillator disposed in a transverse branch which generates anattenuation pole in the blocking range of the filter, a circuit elementdisposed in parallel with the electromechanical oscillator, as a resultof which a parallel resonance frequency lying in the filter blockingrange occurs in said transverse branch, a two-port net disposed in saidcircuit preceding said transverse branch and a twoport net disposed insaid circuit following said transverse branch, said two-port nets eachcontaining at least one longitudinal branch and one transverse branch,with a longitudinal branch of each disposed adjacent the transversebranch containing the electromechanical oscillator, a resonance circuitconnected ahead of the first mentioned two-port net which is disposed ina branch thereat of said filter circuit, and a resonance circuitconnected behind said second mentioned two-port net which is disposed ina branch thereat of said filter circuit, the resonance frequency of saidlast mentioned resonance circuits disposed ahead of and behind saidtwo-port nets corresponding, at least approximately, to the parallelresonance frequency occurring in the filter blocking range of thetransverse branch containing the electromechanical oscillator, suchresonance circuits being so constructed, with respect to the nature ofthe branch in which they are disposed, that they effect a compensationof the parallel resonance frequency occurring in said transverse branchcontaining the electromechanical oscillator.

2. A filter according to claim 1, wherein said resonance circuitsdisposed ahead of and following said two-port nets are like resonancecircuits with identical intrinsic resonance frequency, disposed in likebranches ahead of and following the transverse branch containing saidelectromechanical oscillator.

3. A filter according to claim 2, wherein said resonance circuits areparallel resonance circuits disposed in respective longitudinalbranches.

4. A filter according to claim 2, wherein said resonance circuits areseries resonance circuits disposed in respective transverse branches.

5. A filter according to claim 1, wherein, of said resonance circuitsdisposed ahead of and following said twoport nets, one of said circuitsis a series resonance circuit disposed in a transverse branch, and theother circuit is a parallel resonance circuit disposed in a longitudinalbranch.

6. A filter according to claim 5, wherein the resonance circuit disposedahead of the preceding two-port branch.

7. A filter according to claim 5, wherein the resonance circuit disposedahead of the preceding two-port net is a parallel resonance circuitdisposed in a longitudinal branch.

No references cited.

ELI LIEBERMAN, Primary Examiner. C. BARAFF, Assistant Examiner.

1. AN ELECTRICAL LADDER-TYPE FILTER CIRCUIT HAVING AN ELECTROMECHANICALOSCILLATOR DISPOSED IN A TRANSVERSE BRANCH WHICH GENERATES ANATTENUATION POLE IN THE BLOCKING RANGE OF THE FILTER, A CIRCUIT ELEMENTDISPOSED IN PARALLEL WITH THE ELECTROMECHANICAL OSCILLATOR, AS A RESULTOF WHICH A PARALLEL RESONANCE FREQUENCY LYING IN THE FILTER BLOCKINGRANGE OCCURS IN SAID TRANSVERSE BRANCH, A TWO-PORT NET DISPOSED IN SAIDCIRCUIT PRECEDING SAID TRANSVERSE BRANCH AND A TWOPORT NET DISPOSED INSAID CIRCUIT FOLLOWING SAID TRANSVERSE BRANCH, SAID TWO-PORT NETS EACHCONTAINING AT LEAST ONE LONGITUDINAL BRANCH AND ONE TRANSVERSE BRANCH,WITH A LONGITUDINAL BRANCH OF EACH DISPOSED ADJACENT THE TRANSVERSEBRANCH CONTAINING THE ELECTROMECHANICAL OSCILLATOR, A RESONANCE CIRCUITCONNECTED AHEAD OF THE FIRST MENTIONED TWO-PORT NET WHICH IS DISPOSED INA BRANCH THEREAT OF